<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        :root {
            --common-base-color:aqua
        }
        .container{
            display: flex;
            justify-content: center;
            align-items: center;
        }
        .left {
            width: 100px;
            height: 100px;
            background-color: blue;
        }
        .right {
            width: 100px;
            height: 100px;
            background-color: var(--common-base-color1, black);
        }
        .middle {
            width: 100px;
            height: 100px;
            background-color: var(--common-base-color);
        }
    </style>
</head>
<body>
    <div id="container">
        <div class="left">左盒子</div>
        <div class="right">右盒子</div>
        <div class="middle">中盒子</div>
    </div>
</body>
</html>